The major sources of sucrose for use as white table sugar are sugar beets and sugar cane. The main property that distinguishes refined sugar from raw sugar is its white color. Decolorization, therefore, is a key step in the sugar refining process and in the production of a marketable product. Raw sugar contains non-sugar impurities, such as chromophoric or color forming components and certain trace metals ions including iron, copper, zinc and nickel as well as calcium. These impurities can produce a colored sugar product which is often unacceptable to the consumer. Thus, the impurities must be removed during the decolorization process.
The chromophoric materials in raw sugar typically exist as highly-colored anions, initially in the form of salts of weak acids. However, the chromophoric components may be either highly ionic, weakly ionic, or non-ionic species. The trace metals may either exist as cations, or may be complexed with organic acids or color forming components as anionic complexes. Moreover, the presence of calcium ions can cause scaling during the evaporation of a sugar syrup and can result in the production of poor quality liquid sugar and sugar crystals.
The sugar refining industry, particularly the cane sugar refining industry, traditionally decolorizes sugar by using carbon adsorbents as the principal method of decolorization. Carbon adsorbents are well known in the art and typically include powdered carbons, bone char (carbonized bone particles) and granular carbon. Many of the sugar colorants are, therefore, anionic in nature and can be removed from solution by ion exchange materials. Some refineries have replaced carbon adsorbents in whole or in part with anion-exchange resins. Calcium ions are removed from sugar syrup with cation exchange resins.
During the decolorizing of sugar syrup, particularly from sugar cane, with activated carbons and anion exchange resins, the pH value of the syrup drops substantially below pH 7. It is not uncommon, therefore, for the acidity in a sugar syrup to reach a pH value as low as 1.5 to 2.5. It is well known that acidic conditions promote the hydrolysis of a disaccharide, such as sucrose, to its corresponding monosaccharide units, fructose and glucose, in a process known as inversion. When a sugar syrup is warmed, such as during a filtration process, the rate at which inversion takes place increases.
Unless the acidity of sugar syrup is neutralized, a substantial loss in sucrose yield can occur as a result of inversion. Consequently, during the industrial clarification of sugars, commonly referred to in the art of sugar refining as "defecation," alkalizing agents including magnesium oxide and calcium oxide (lime), are generally added to a sugar syrup to maintain the pH of the sugar syrup at a value greater than pH 7. This procedure, however, is costly and adds uncontrolled quantities of metal cations to the treated syrup.
Carbon adsorbents are general adsorbents. As a result, they adsorb most materials from sugar syrup, including sugar, with little or no selectivity. Powdered carbons can only be used once or twice and are expensive. Granular carbon has no ion-exchange properties, does not remove ash, and must contain magnesium carbonate for pH control. Bone char has ion-exchange properties and removes considerable ash from the sugar. Bone char comprises about 6 to 10 percent carbonaceous residue and about 90 percent calcium phosphate supplied by the degreased cattle bones from which it is prepared. Consequently, bone char does provide a buffering effect that keeps the pH value of the sugar syrup from dropping and is regenerable on heating. However, the buffering capacity of bone char initially is low and decreases with use. Further, the use of bone char requires a substantial capital investment in plant equipment, uses considerable energy resulting in a high cost of fuel for the kilns used to regenerate the bone char and increases the cost of sweet water evaporization. Conventional ion-exchange processes using regenerable deep beds of ion-exchange resins for decolorization are also costly, generate substantial amounts of sweet water, require large volumes of hot rinse water and may pose chemical waste problems.
It is commercially desirable to remove non-sugar contaminants from a sugar syrup prior to the defecation step because liming (the addition of calcium oxide to the syrup to control the pH) can increase the ash content. Thus, a need exists to minimize or eliminate the defecation step by eliminating non-sugar components early in the process before these components adversely affect the color and quality of the crystallized sugar. It may also be necessary to decolorize final syrups prior to crystallization of the sugar. Here again, pH control is important.